1. Field of the Invention
The present invention generally relates to an electrophotographic color image forming apparatus and an image forming method, and more particularly to techniques suitable for a color offset printing system and an image forming apparatus using laser writing/silver halide imaging.
2. Description of the Related Art
Image data to be input to an image forming apparatus has multivalued data ranging from 8 to 12 bits per pixel in gradation images such as photographs. By contrast, the image forming apparatus (including an electrophotographic image forming apparatus) for forming an image on paper (namely, hard copy) practically has a very small number of gradations that can be expressed per pixel. In order to solve such a problem, hard-copy devices increase resolution thereof to 600 dpi or 1200 dpi, for example, and display pseudo-halftone images by modulating image density in terms of area using plural pixels. Pseudo-halftone processing refers to image processing performed by converting input image data to a pseudo-halftone image.
Details of processing for quantizing the multivalued image data in the dither method are described in Electrophotography, Vol. 24, No. 1 (1985) p. 51-59, for example, so that the description is omitted in the present specification. Images subjected to dither processing have periodic structures. Further, color images are formed by superposing plural toner images (full-color images are generally formed by superposing four colors, namely, Y: yellow, C: cyan, M: magenta, and K: black). In addition, these images of four colors are subjected to sets of dither processing differing to one another and the toner images have different periodic structures. In the past, in some cases, the images of four colors are subjected to dither processing such that the toner images have the same periodic structure. However, in such cases, colors tend to change depending on fluctuation of positions where colors are superposed, so that this type of dither processing is not usually used today. Nowadays, a method usually used is capable of avoiding the generation of such a problem and the images of four colors are processed to have different periodic structures (screen ruling and screen angles are shifted). This method has been widely used in the field of printing and followed by the field of hard copying other than printing such as electrophotography.
Types of dither matrices are classified into (1) Dot screen type, (2) Bayer type, and (3) Line screen type. In comparison with the dot screen type dither matrices, the line screen type dither matrices have the following merits. In the dot screen type dither matrices, the periodic structure of a growth center is required to have a substantially square shape, so that a degree of freedom for screen ruling and screen angles defining the dither matrices is limited. By contrast, in the line screen type dither matrices, the periodic structure of a growth center may be a rectangle or a parallelogram without causing any difference in comparison with the case of the square shape, so that it is possible to increase the number of combinations (degree of freedom in selection) of the screen ruling and the screen angles.
In the method for superposing toner images having different periodic structures regarding the images of four colors, a phenomenon may be generated as if waves were superposed and interference patterns referred to as beat may be observed. The interference patterns are referred to as “color moiré”. When the color moiré is generated in a low frequency area (frequency of beat is low) and visually recognized, the color moiré brings discomfort to a user and may become a factor in degradation of image quality. Usually, a combination method is selected such that this color moiré generated when the images of four colors are superposed is not visually noticeable as much as possible (frequency of moiré is high) and dither matrices used for dither processing in four colors are determined. However, there is no established method for maintaining balance of color moiré for all colors (superposition of all the CMYK colors), so that combinations empirically determined as suitable are widely employed.
Currently, combinations of dither matrices widely used for four colors are determined by a method for arranging the four colors of C, M, Y, and K widely employed for industrial printing apparatuses as shown in FIGS. 2 and 3. In this method, screen angles are set such that Y color is set to have a screen angle of 0 degree, C color is 15 degrees, K color is 45 degrees, and M color is 75 degrees (although screen ruling is not limited, 175 lpi or so is used for the each of the CMYK colors in the same manner). In order to strictly realize the screen angles, a resolution of not less than 2400 dpi is necessary, so that when the resolution is less than 2400 dpi, other screen angles which can be realized and are close to the above-mentioned screen angles are selected (FIG. 3 corresponds to a case where the above-mentioned screen arrangement is formed using the resolution of 2400 dpi). Further, in this arrangement method, the periodic structure has a square shape and halftone dot shapes are based on the dot screen type, so that an angle shifted by 90 degrees relative to each screen angle has an equivalent directional axis. In this combination, by making use of the fact that color moiré generated between Y color and CM colors is not noticeable, difference of screen angles between Y color and CM colors is set as 15 degrees (in the field of printing, color moiré generated between Y color and other CMK colors is considered to be small). In addition, well-known techniques to eliminate color moiré generated by superposing toner images of plural colors with periodic structures include Japanese Laid-Open Patent Application No. 2002-112047, for example.
However, it is difficult to determine a combination of dither matrices for the four CMYK colors (hereafter, a combination of dither matrices for the four colors is referred to as a dither set) since there is no established method as mentioned above. In particular, it is very difficult to develop a preferable dither set other than empirically known dither sets.
An optical writing device of an electrophotographic image forming apparatus scans a photoconductor and forms an electrostatic latent image in which a beam is irradiated from a semiconductor laser, optically modulated, and reflected on a face of a polygon mirror rotating at a high speed. In this scanning method using the polygon mirror, it is known that a beam position on the photoconductor is changed in what is called a sub-scanning direction (movement direction of the rotating photoconductor). This problem is generally referred to as “surface tilt”. Causes of the surface tilt include a tilt of a face of the polygon mirror relative to a motor rotating shaft, fluctuation of a tilt of each face of the polygon mirror (tilt relative to a rotating shaft, namely, a surface tilt angle), or the like. For example, Japanese Laid-Open Patent Application No. 2003-260813 discloses measures for surface tilt.
Usually, in order to reduce the above-mentioned surface tilt, an optical system of the optical writing device is disposed such that there is a conjugate relationship between faces of the polygon mirror and a face of a photoconductor drum. This method is generally known as a function for preventing surface tilt. However, even when the surface tilt preventing function is used, a position of a beam incident on the polygon mirror is different depending on a scanning position (position in a main scanning direction), so that it is difficult to completely eliminate the change of a writing position in the sub-scanning direction resulting from the surface tilt. As a result, the change of a writing position is generated. In accordance with the surface tilt, an uneven pitch (unevenness of scanning intervals in the sub-scanning direction) is generated, which has a space period based on the number of faces of the polygon mirror in the sub-scanning direction. In this case, an uneven density corresponding to the unevenness of pitch is generated in an image. This is recognized by the user (when the pitch of the uneven density is large and visually noticeable) and becomes a factor in degradation of image quality as an abnormal image.
A method for eliminating color moiré disclosed in Japanese Laid-Open Patent Application No. 2002-112047 employs a method for shifting only those phases while using the same periodic structure (the same screen ruling and screen angles) for Y color and one of CMK colors. However, this method is available only when positional accuracy of superposed colors is high, so that when positional displacement of the superposed colors is large (about 10% of a single period of dithering), a tone of an output image is changed because of the positional displacement of the superposed colors. In other words, differing from the above-mentioned method by which the change of a tone is eliminated by shifting screen angles, this method is not a fundamental solution to the positional accuracy of the superposed colors.
Further, although a method disclosed in Japanese Laid-Open Patent Application No. 2003-260813 is assumed to be preferable in terms of color moiré, the number of combinations of dithering for the CMYK colors is two and other combination is not disclosed, for example. Further, Japanese Laid-Open Patent Application No. 2003-260813 discloses no description regarding what combination is preferable in terms of color moiré.
In accordance with the above-mentioned description, it is understood that there is no established method for determining a combination for the four CMYK colors so as not to generate color moiré and that it is very difficult to develop a dither set preferable in terms of color moiré.
As mentioned above, by using line screen type dither matrices in order to constitute the dither set, it is possible to increase the number of combinations (degree of freedom in selection) of the screen ruling and the screen angles (since the degree of freedom is improved, possibilities of combination preferable in terms of color moiré are increased). In the case of the dot screen type dithering, it is necessary to set directional axes (axes parallel with vectors indicating periodic structures) for the four CMYK colors in a range of 90 degrees, since the directional axes are present at intervals of 90 degrees in the dot screen type dither matrices. By contrast, in the line screen type dither matrices, it is sufficient to set directional axes for the four CMYK colors in a range of 180 degrees, since the directional axes are present at intervals of 180 degrees in the line screen type dither matrices. Thus, it is possible to have larger screen angles among the CMYK colors in comparison with the dot screen type dithering. As a result, the line screen type dithering is more likely to obtain images preferable in terms of moiré.
However, upon selecting dither sets, it is very difficult to try all combinations because there are tens of thousands of combinations. Thus, in practice, combinations of dither matrices (referred to as dither sets) for the CMYK colors empirically determined as suitable are used. In other words, it is not easy to select a combination preferable in terms of color moiré by combining the line screen type dithering.